Consider helping financially, especially with the cost of medications and treatments. Psychiatric medication can cost thousands of dollars every month. Insurance companies tend to approach coverage differently for mental disorders, and necessary treatments aren't always covered. Many people affected by mental illness lose their jobs or are disabled for a time, with obvious financial implications.
Establish your personal boundaries and stick to them. Are you willing for group members to call you at home? Visit your home? How much time can you spend doing direct ministry outside your group meetings? Answer questions like these, tell your group about your boundaries, and apply them consistently with everyone in the group. Then stick to them. If someone violates your personal boundaries or demands more of you than you can give, grant the person the dignity of restating your boundaries rather than just ignore the person or drop out of his or her life.
Model acceptance. Set an example of acceptance during your group prayer times by sometimes praying for people affected by mental illness, just as you do for people affected by other forms of illness and injury. Without sharing any confidential information, mention people struggling with mental illness in your prayer requests—maybe someone in your family or workplace is suffering, or make your request general. Do this without passing judgment or giving commentary on people's illness. This ministry can go a long way toward creating a feeling of safety and hope.
As a group, decide what kind of behavior you will tolerate. Make those expectations very clear to everyone, and establish what action you'll take if someone violates those group expectations. Then if a person's behavior becomes a problem, follow the established process. But be sure to make clear you are addressing the person's behavior, rather than the mental illness. Don't assign an amateur diagnosis, and do not suggest that the person needs to "fix themselves" before being accepted into the body of Christ. Simply restate the group's expectations and your requirement that they be honored. Please also be gracious when forming the expectations, allowing for some latitude in social skills and focusing on what's truly important.
Work through a Bible study on mental illness. SmallGroups.com offers a great study called Ministering to Those with a Mental Illness, or read a mental-health-related book (such as my book Troubled Minds) together and discuss it. Mental illness is rarely addressed in sermons, church classes, and small groups. Bring the issue out into the open and give people permission to discuss it while wrestling through their theological questions.
Get some training. Encourage your church to host a small-group training event for all small-group leaders, perhaps through FaithNet programs, sponsored by the National Alliance on Mental Illness (NAMI).
Pray together. If someone in your group is affected by mental illness and gives permission, pray for that person as a group. But please do this in a way that does not suggest that prayer is a substitute for mental-health treatment. Simply demonstrate that you care, that you know God cares, and that you believe prayer is powerful in the life of every suffering person—and an important part of every pursuit of healing.